Examples of conventional electric motors of this type are shown for example in Japanese Utility Model Kokai Publication No. 135863/1985 and Japanese Utility Model Kokai Publication No. 104779/1986. These will be explained with reference to FIG. 1, FIG. 2, and FIG. 3.
Referring first to FIG. 1 and FIG. 2, a magnetic disk 1 for recording information is held by a hub 16, which is fitted with and fixed to a rotary shaft 17. A magnetic disk 1 comprises a recording sheet 1a, a center hub 1d connected to the recording sheet 1a, outer case comprising an upper wall 1b and a lower wall 1c for protecting the sheet 1a.
The center hub 1d of the magnetic disk 1 is provided with an aperture 1e through which an upper end of the rotary shaft 17 extends. A drive pin 15 is fixed to the hub 16 at a certain distance from the axis of the shaft 17, and is engageable with an aperture if provided in the center hub 1d of the magnetic disk 1. The drive pin 15 transmits the rotation of the shaft 17 and the hub 16 to the magnetic disk 1. In this way, the hub 16 and the drive ping 15 in combination hold and rotate the magnetic disk 1.
The rotary shaft 17 is rotatably supported with a pair of ball-bearings 11 at both axial ends of the cylindrical bearing holder 19. Fixed to the outside of the bearing holder 19 is a stator 20 having a core 8, which comprises an annular part 8a and a plurality of tooth portions 8b, on which coils 21 are wound. The core 8 and the cylindrical bearing holder 19 are fixed to a frame 18 by means of screws 14.
Fixed by a screw 6 to a first (lower as seen in FIG. 1) end of the rotary shaft 17, the end opposite to the hub 16 is a generally cup-shaped yoke 23 having a disk-shaped bottom wall 23a and a cylindrical side wall 23b having its first (lower as seen in FIG. 1) end connected to the outer periphery of the disk-shaped bottom 23a. Fixed to the inner surface of the side wall 23b of the yoke 23 is a driving permanent magnet 22 having magnetic poles facing the outer ends of the tooth portions 8b of the core 8. The yoke 23 and the permanent magnet 22 form a rotor 24.
The driving permanent magnet 22 emanates magnetic lines of force radially inward. By the interaction of the magnetic lines of force and the electric current through the coils 21, a rotating torque is generated, by which the rotor 22 is rotated.
An index detecting element 25 generates, in cooperation with an index detecting magnet 26, a pulse every revolution.
As shown in FIG. 3, a signaling permanent magnet (FG (frequency generator) magnet) 27 for detecting the rotary angle and the rotating speed (the number of revolutions per a unit time) of the rotor 24 may be mounted to an end of the driving permanent magnet 22, at the upper edge of the side wall 23b of the yoke 23. The frame 28 of this example of FIG. 3 also serves as a circuit board, on which a detecting circuit pattern 29 is formed at the location confronting the signaling permanent magnet 27. The circuit pattern 29 has a size substantially equal to the signaling permanent magnet 27, and generates an FG output signal in time with the rotation of the rotor 24.
The above-described electric motor is of the outer-rotor type in which the driving permanent magnet 22 of the rotor 24 is positioned outside the stator 20. The magnetic flux from the index detecting magnet 26 is detected by the index detecting element 25, and on the basis thereof, the current through the coils 21 is controlled.
Magnetic heads 41 and 42 are mounted to the frame 18 by means of a mounting structure, which includes a supporting shaft 43 and a sliding block 44. They are so supported that they are capable of translation movement in the radial direction of the magnetic disk 1.
In the above-described conventional electric motor, the rotor 24 is on the outside so when it is contemplated to reduce the thickness of the electric motor and the magnetic recording device in which the electric motor is built, magnetic interferences between the permanent magnet 22 of the rotor 24, and the magnetic heads 41 and 42 and other components of the recording device problematical. The interferences can be eliminated or reduced by use of the inner rotor type in which the rotor 24 is positioned inside the stator 20, but the conventional FG detecting system shown in FIG. 3 cannot be adapted in the electric motor of the inner-rotor type.
Moreover, since the magnetic heads 41 and 42 move in the radial direction of the magnetic disk, they cannot be disposed in the plane of the rotor 24. This also imposes a limitation to the reduction of the thickness of the magnetic recording device.
Furthermore, the frame 18 and the bearing holder 19 are interposed between the stator 20 and the rotor 24 on one hand, and the magnetic disk and the magnetic heads, on the other hand. This imposes a further limitation to the reduction in thickness of the electric motor. If an electric motor of the inner-rotor type is employed the above problem can be solved. But, the rotation of the rotor 24 is accompanied with the so-called thrust force in the direction of the rotary shaft. When the thrust force is excessive, the entire rotor is shifted upward in the direction of the rotary shaft. This makes it impossible to rotate the magnetic disk to rotate at a proper height (position along the shaft), adversely affecting the recording and playback.